1. Field of the Invention
The present invention relates to interferometric fiber optic gyros (IFOG), and more particularly to improving the total gyro linearity in an IFOG.
2. Description of the Related Art
There is a growing demand for high accuracy gyros for satellite pointing applications. In order to improve gyro sensitivity, it is necessary to lower the total gyro noise. In typical satellite pointing applications, an interferometric fiber optic gyro (IFOG) is employed. In the IFOG, noise elements arise from both the optical and electrical elements.
A closed loop IFOG is illustrated in FIG. 1. An IFOG generally includes a light source 10, a coupler 20, an integrated optics chip (IOC) 30, and a fiber coil 40, which comprise the optical circuit 5. The fiber coil 40 provides the rotation-sensitive interferometer. The processing electronics 45 of the IFOG generally comprise a photodetector 50, an amplifier/filter 60, an analog-to-digital converter (A/D) 70, a digital signal processor (DSP) 80, a digital to analog converter (D/A) 90 and amplifier 95.
The processing electronics 45 function to provide a feedback phase shift in the optical circuit 5 which effectively compensates a rotation-induced phase shift sensed in the fiber coil 40. The magnitude of the feedback phase shift is an indication of the rotation rate. The photodetector 50 converts an optical power output by optical coupler 20 to a corresponding voltage. The corresponding voltage is processed by amplifier/filter 60 and converted to a digital signal by A/D converter 70. A corresponding feedback signal is calculated in DSP 80, and fed back into the gyro via D/A converter 90 and amplifier 95.
A rotation about a rate input axis 41 of the fiber coil 40 produces input signals at the photodetector 50, which are denoted as xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d in FIG. 1A. A difference A-B corresponds to the sensed input rotation rate and is designated Derror. The input signal characteristics result from interference patterns of a counter within the light source 10, which propagates square wave modulated signals that travel in the fiber optic coil 40. The sharp spikes present in the waveform are a result of the modulated signals driving the interference patterns through the peak of the interference curves.
An accurate measurement of the magnitude of the A and B levels of the photodetected signals is crucial to the overall performance of the IFOG. However, this measurement accuracy is compromised by the presence of noise and optical spikes. A high gain is required to maintain closed loop performance. However, the high gain results in signal distortion due to saturation effects from the optical spikes in the signal processing electronics 45, most notably in the photodetector 50 and amplifier/filter 60.
Noise generated within the photodetector 50 also makes detection of the A and B levels difficult, which results in degraded performance within the IFOG.
A conventional photodetector 50 is further illustrated in FIG. 2. Referring to FIG. 2, the photodetector 50 is comprised of the photodiode 52, amplifier 54, and feedback resistor 55. The photodiode 52 is reverse biased to operate in a photoconductive mode, thereby optimizing the bandwidth and speed of the photodetector 50 while minimizing signal distortion.
In operation, the photodiode 52 converts optical power, received from the coupler 20, into a corresponding electrical current denoted as Iph. The current flows through feedback resistor 55. In this configuration, the amplifier 54 provides an output voltage Vout, which is calculated using Equation 1 below:
VOUTxe2x89xa1xe2x88x92Iphxc2x7Rfxe2x80x83xe2x80x83Equation 1
where Rf is the value of the feedback resistor.
The amplifier 54 provides amplification while maintaining a high input impedance with respect to the photodiode 52. The photodiode 52 and amplifier are selected based on parameters, such as bandwidth, amount of detected power, optical wavelength, available gain, etc. The photodetector 50 circuitry is typically realized in a hermetically sealed microcircuit package for ruggedness and shielding from external electric fields, as is commonly known in the art.
A disadvantage of the prior art photodetector 50, however, is the susceptibility of the photodiode 52 and feedback resistor 55 to thermal noise. As discussed above, a relatively high gain is required to maintain closed loop performance. Consequently, large resistance values are required in the photodiode 52 (equivalent resistance) and feedback resistor 55. This results in the generation of thermal noise in the photodiode 52 and feedback resistor 55. Thermal noise, also known as Johnson noise, is a well-documented phenomenon in which electronic noise signals are produced by the random thermal motion of charges in circuit elements. Thermal noise varies as a function of resistance and temperature. Thermal noise is a major contributor to the noise generated within the photodetector 50, which results in a degradation of IFOG performance, as discussed above.
Therefore, there is a need for a photodetector with reduced thermal noise generation.
It is therefore an object of the present invention to provide a photodetector having reduced thermal noise generation.
In accordance with an aspect of our invention the photodetector of an Interferrometric Fiber Optic Gyro (IFOG) includes a photodiode that converts an optical power signal received from a coupler of the IFOG to an electrical compensation signal and which photodiode is in mechanical contact with one or more thermal electrical coolers (TEC) to lower an operating temperature of the photodiode. The photodetector also includes an amplifier circuit to amplify the electrical compensation signal. The amplifier circuit includes an operational amplifier (OP-AMP) having an input and an output, with a feedback resistor interposed between the input and output. The feedback resistor is also in mechanical contact with a TEC to lower an operating temperature of the feedback resistor. In accordance with our invention by reducing the operating temperature of the feedback resistor and photodiode the thermal noise of the IFOG is also reduced.